1. Field of the Invention
The present invention relates to a blanking circuit for use in a display apparatus having a cathode-ray tube (CRT), such as a television receiver, a terminal display, or a projection-type television receiver.
2. Description of the Related Art
As is known, a display apparatus having a CRT is provided with a blanking circuit for cutting off the electron beam during the horizontal and vertical blanking periods. The blanking circuit is designed to change the potential difference between the cathode and the screen grid of the CRT, during the blanking periods from the potential difference during the horizontal and vertical scanning periods when the potential difference between the cathode and the screen grid is so changed, the electron beam emitted from the cathode is cut off and prevented from reaching the phosphor layer coated on the inner surface of the face plate of the CRT. More specifically, the circuit increases the cathode potential, or decreases the potential of the screen grid, thereby cut off the CRT during the horizontal and vertical blanking periods.
The conventional blanking circuit, which is designed to decrease the potential of the screen grid, includes a blanking pulse amplifier. The blanking pulse amplifier is connected to the screen grid by means of a first resistor and a capacitor. The node of the first resistor and the capacitor is connected by a second resistor to the tap of a variable resistor, one end of which is coupled to a reference voltage terminal, and the other end of which is connected to the ground. Hence, the voltage generated at the tap is applied to the screen grid via the second resistor and the first resistor, as the voltage (i.e., the luminance-adjusting voltage) which is to determining the voltage to be applied between the cathode and the grid during each scanning period.
The blanking pulse amplifier comprises a transistor. The transistor amplifies a blanking pulse signal, and its collector output is supplied to one end of the capacitor. The collector of the transistor is connected by means of the third resistor to a power-supply terminal, and the emitter of the transistor is connected by means of a fourth resistor to the ground.
The blanking pulse signal supplied to the base of the transistor of the blanking pulse amplifier is set at a low voltage level during every scanning period. During the scanning period, the transistor remains in an "off" mode, and the high-level voltage is applied to the screen grid from the variable resistor. During every blanking period, the blanking pulse signal is set at a high level, and the transistor remains in the "on" mode, whereby the low-level voltage determined by the resistances of the third and fourth resistors is applied to the screen grid, hence, the electron beam is cut off. This operation is called "retrace blanking".
When a high-frequency pulsating signal is supplied to the cathode of the CRT, a signal having an identical polarity to the input signal is induced in the screen grid, due to the capacitive coupling between the cathode and the screen grid. For example, when a negative signal is induced in the screen grid, the potential of the grid falls, and the luminance of the CRT screen decreases and remains at a low value as long as the grid potential stays at a low level. In other words, whenever the potential of the screen grid fluctuates, the luminance of the CRT screen inevitably varies. If this is the case, image data can no longer be displayed in the desired luminance.
In order to prevent the grid potential from fluctuating, a capacitor can be connected between the screen grid and the ground, for removing a ripple from the potential of the screen grid. The greater capacitance the ripple-removing capacitor has, the better. However, when use is made of a capacitor having too great of a capacitance, the blanking pulses supplied to the screen grid will have distorted waveforms. Consequently, it become impossible to cut off the electron beam during the horizontal and vertical blanking periods. The images displayed on the CRT screen will inevitably be unclear, particularly in those portions close to the edge of the CRT screen.